Improve crypto ordering for Encrypt-then-MAC (EtM) mode MAC algorithms.

Previously we were computing the MAC, decrypting the packet and then
checking the MAC. This gave rise to the possibility of creating a
side-channel oracle in the decryption step, though no such oracle has
been identified.

This adds a mac_check() function that computes and checks the MAC in
one pass, and uses it to advance MAC checking for EtM algorithms to
before payload decryption.

Reported by Jean Paul Degabriele, Kenny Paterson, Torben Hansen and
Martin Albrecht. feedback and ok markus@

diff --git a/usr.bin/ssh/mac.c b/usr.bin/ssh/mac.c
index b0b4362..ed12dd4 100644 (file)
--- a/usr.bin/ssh/mac.c
+++ b/usr.bin/ssh/mac.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: mac.c,v 1.32 2015/01/15 18:32:54 naddy Exp $ */
+/* $OpenBSD: mac.c,v 1.33 2016/07/08 03:44:42 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  *
@@ -159,7 +159,8 @@ mac_init(struct sshmac *mac)
 }
 
 int
-mac_compute(struct sshmac *mac, u_int32_t seqno, const u_char *data, int datalen,
+mac_compute(struct sshmac *mac, u_int32_t seqno,
+    const u_char *data, int datalen,
     u_char *digest, size_t dlen)
 {
        static union {
@@ -203,6 +204,24 @@ mac_compute(struct sshmac *mac, u_int32_t seqno, const u_char *data, int datalen
        return 0;
 }
 
+int
+mac_check(struct sshmac *mac, u_int32_t seqno,
+    const u_char *data, size_t dlen,
+    const u_char *theirmac, size_t mlen)
+{
+       u_char ourmac[SSH_DIGEST_MAX_LENGTH];
+       int r;
+
+       if (mac->mac_len > mlen)
+               return SSH_ERR_INVALID_ARGUMENT;
+       if ((r = mac_compute(mac, seqno, data, dlen,
+           ourmac, sizeof(ourmac))) != 0)
+               return r;
+       if (timingsafe_bcmp(ourmac, theirmac, mac->mac_len) != 0)
+               return SSH_ERR_MAC_INVALID;
+       return 0;
+}
+
 void
 mac_clear(struct sshmac *mac)
 {

diff --git a/usr.bin/ssh/mac.h b/usr.bin/ssh/mac.h
index e5f6b84..0b119d7 100644 (file)
--- a/usr.bin/ssh/mac.h
+++ b/usr.bin/ssh/mac.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: mac.h,v 1.9 2015/01/13 19:31:40 markus Exp $ */
+/* $OpenBSD: mac.h,v 1.10 2016/07/08 03:44:42 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  *
@@ -46,6 +46,8 @@ int    mac_setup(struct sshmac *, char *);
 int     mac_init(struct sshmac *);
 int     mac_compute(struct sshmac *, u_int32_t, const u_char *, int,
     u_char *, size_t);
+int     mac_check(struct sshmac *, u_int32_t, const u_char *, size_t,
+    const u_char *, size_t);
 void    mac_clear(struct sshmac *);
 
 #endif /* SSHMAC_H */

diff --git a/usr.bin/ssh/packet.c b/usr.bin/ssh/packet.c
index b03d3f4..7e45553 100644 (file)
--- a/usr.bin/ssh/packet.c
+++ b/usr.bin/ssh/packet.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: packet.c,v 1.230 2016/03/07 19:02:43 djm Exp $ */
+/* $OpenBSD: packet.c,v 1.231 2016/07/08 03:44:42 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -1678,7 +1678,7 @@ ssh_packet_read_poll2(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)
 {
        struct session_state *state = ssh->state;
        u_int padlen, need;
-       u_char *cp, macbuf[SSH_DIGEST_MAX_LENGTH];
+       u_char *cp;
        u_int maclen, aadlen = 0, authlen = 0, block_size;
        struct sshenc *enc   = NULL;
        struct sshmac *mac   = NULL;
@@ -1778,17 +1778,21 @@ ssh_packet_read_poll2(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)
         * 'maclen' bytes of message authentication code.
         */
        if (sshbuf_len(state->input) < aadlen + need + authlen + maclen)
-               return 0;
+               return 0; /* packet is incomplete */
 #ifdef PACKET_DEBUG
        fprintf(stderr, "read_poll enc/full: ");
        sshbuf_dump(state->input, stderr);
 #endif
-       /* EtM: compute mac over encrypted input */
+       /* EtM: check mac over encrypted input */
        if (mac && mac->enabled && mac->etm) {
-               if ((r = mac_compute(mac, state->p_read.seqnr,
+               if ((r = mac_check(mac, state->p_read.seqnr,
                    sshbuf_ptr(state->input), aadlen + need,
-                   macbuf, sizeof(macbuf))) != 0)
+                   sshbuf_ptr(state->input) + aadlen + need + authlen,
+                   maclen)) != 0) {
+                       if (r == SSH_ERR_MAC_INVALID)
+                               logit("Corrupted MAC on input.");
                        goto out;
+               }
        }
        if ((r = sshbuf_reserve(state->incoming_packet, aadlen + need,
            &cp)) != 0)
@@ -1798,26 +1802,21 @@ ssh_packet_read_poll2(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)
                goto out;
        if ((r = sshbuf_consume(state->input, aadlen + need + authlen)) != 0)
                goto out;
-       /*
-        * compute MAC over seqnr and packet,
-        * increment sequence number for incoming packet
-        */
        if (mac && mac->enabled) {
-               if (!mac->etm)
-                       if ((r = mac_compute(mac, state->p_read.seqnr,
-                           sshbuf_ptr(state->incoming_packet),
-                           sshbuf_len(state->incoming_packet),
-                           macbuf, sizeof(macbuf))) != 0)
+               /* Not EtM: check MAC over cleartext */
+               if (!mac->etm && (r = mac_check(mac, state->p_read.seqnr,
+                   sshbuf_ptr(state->incoming_packet),
+                   sshbuf_len(state->incoming_packet),
+                   sshbuf_ptr(state->input), maclen)) != 0) {
+                       if (r != SSH_ERR_MAC_INVALID)
                                goto out;
-               if (timingsafe_bcmp(macbuf, sshbuf_ptr(state->input),
-                   mac->mac_len) != 0) {
                        logit("Corrupted MAC on input.");
                        if (need > PACKET_MAX_SIZE)
                                return SSH_ERR_INTERNAL_ERROR;
                        return ssh_packet_start_discard(ssh, enc, mac,
                            state->packlen, PACKET_MAX_SIZE - need);
                }
-
+               /* Remove MAC from input buffer */
                DBG(debug("MAC #%d ok", state->p_read.seqnr));
                if ((r = sshbuf_consume(state->input, mac->mac_len)) != 0)
                        goto out;